Cancer, the uncontrolled growth and spread of malignant cells that can affect almost any tissue of the body, continues to be a major burden to public health. Anticancer chemotherapy is the most important adjuvant therapy, but the targeted and effective therapy to the tumorous tissues or organs, however, remains as a significant challenge in chemotherapy. The proposed work will develop molecular nanofibers of small molecules for controlling the behavior and the fate of cells. The goal of this work is to explore cellular responses to enzyme-instructed formation of molecular nanofibers and eventually develop new nanomedicines to target cancer cells. This application is both hypothesis and design driven. We hypothesize that enzyme- instructed molecular self-assembly, as a unique way to create molecular nanofibers, can modulate extra- and intracellular microenvironment and selectively lead to death of cancer cells. To validate the hypothesis, we will design molecules that self-assemble to form nanofibers upon the action of extra- or intracellular enzyme(s), characterize the physiochemical properties of the nanofibers, and assess the biological properties and effects of the formation of molecular nanofibers in vitro and in vivo. Specifically, this proposed research will (i) design and synthesize substrates that can be converted into molecular nanofibers by enzyme catalysis; (ii) characterize the enzyme-catalyzed reactions of the designed precursors and the behavior and properties of the corresponding molecular nanofibers; (iii) evaluate the activity of the molecular nanofibers against representative cancer cell lines in vitro; and (iv) examine formation and anticancer effects of the molecular nanofibers in vivo using animal models. This research will potentially provide a new platform for creating synthetic nanostructures as nanomedicine to target cancer cells. We anticipate that this new approach will improve fundamental understanding of cancer therapy, provide guiding principles to design anticancer agents at nanoscale, and ultimately lead to a new paradigm of cancer therapy that are based on the integration of molecular self-assembly and enzyme catalysis.